3 Development of Orographic Clouds The first requirement is a topographic barrier, a mountain range. In this example, west is on the left, and east, on the right. We assume zonal (west-to-east) flow will be the typically force cloud development. W IN THE ATMOSPHERE Higher = colder Lower = warmer E

4 W Development of Orographic Clouds If the flow is strong enough, air is forced up and over the mountain barrier, rising, and cooling. Note that in the lee (on the east side), the air descends and warms. IN THE ATMOSPHERE Higher = colder Lower = warmer E

5 Development of Orographic Clouds When the air is sufficiently moist, condensation occurs, and a cloud of tiny water droplets forms. These drops are too small to precipitate. The cloud remains ice-free even at temperatures below 0 C. This supercooling slows the precipitation process. IN THE ATMOSPHERE Higher = colder Lower = warmer

6 Supercooled Water When solid substrates contact the supercooled droplets, they freeze. This accretion builds up on the exposed substrate. This results in aircraft icing. Automobile Radio Antenna

10 Development of Orographic Precip Once ice forms, a race begins for the newly-formed ice to grow large enough to precipitate before beginning the descent and warming in the lee. IN THE ATMOSPHERE Higher = colder Lower = warmer GROWTH ZONE. First Ice

11 Development of Orographic Precip Additional ice crystals grow large enough to become snow (and may melt, becoming rain) as they travel beyond the crest. But in the descending, warming cloud, many remain too small to fall. Others melt and evaporate. First Snow or Rain

12 Development of Orographic Precip Cloud seeding provides additional ice nuclei that create ice at warmer temperatures. This causes cloud ice to form sooner, at temperatures as warm as -8 C. (Remember: the natural ice nuclei start to make ice between -15 to -20 o C.) GROWTH ZONE, SEEDED. GROWTH ZONE, UNSEEDED. First Ice, seeding First Ice, not seeded

13 Development of Orographic Precip Precipitation is thus increased because more ice More ice crystals grow large enough to crystals have a better opportunity to grow large become snow as they travel beyond the peak, enough, becoming snow while over the mountain. but in the descending, warming cloud, many are Cloud too seeding small converts to fall. Others supercooled may liquid melt cloud and water into snow, increasing cloud efficiency. This evaporate. cloud water would otherwise often simply pass by. GROWTH ZONE, SEEDED

14 How Do We Seed Effectively?

15 How Do We Seed Effectively? (1) We must recognize when conditions are favorable. Are clouds present? Are they cold enough? Do they contain liquid water? Can seeding agent be delivered effectively?

16 How Do We Seed Effectively? (2) Creation of Ice Nuclei. From the right place - within cloud, or from a location where it will reach supercooled cloud, The right amount, that is, enough to make a difference.

17 How Do We Seed Effectively? (3) Movement from the release points to the supercooled cloud. If the seeding agent (ice nuclei) are not placed directly within the supercooled cloud, nature must transport it. Dispersion dilutes and spreads the seeding agent.

18 How Do We Seed Effectively? (2 and 3) Together these links are often termed TARGETING. Many programs have failed because targeting was poor.

19 How Do We Seed Effectively? (4) As seeding agent enters supercooled cloud (especially between -8 and -20 C), the rate of ice crystal formation and the number of ice crystals increase.

20 How Do We Seed Effectively? (5) The newly-created ice crystals grow, converting the tiny cloud droplets to much larger ice crystals, heavy enough to precipitate.

21 How Do We Seed Effectively? (6) The ice crystals created by seeding are now numerous and precipitating. If our targeting was correct, these crystals will reach the surface as snow, or with warmer temperatures below, rain. If we released enough seeding agent, and there was enough supercooled liquid, precipitation will be increased.

22 How Do We Seed Effectively? (7) Evaluation. Links (1) through (6) may generate more precipitation, but to know how much we must measure it, usually both within and outside of the seeded area, and during seeded and non-seeded periods. The evaluation topic is complex, and not dealt with in this presentation.

23 1. The cloud must contain supercooled liquid water (SLW), so is the cloud cold enough? 2. Is the wind flow over the barrier, not around it? 3. Will there be enough time for the ice produced by seeding to grow large enough to precipitate? If (1) and (2) are TRUE, we must do (3) such that it also will be TRUE. This is targeting. Targeting

24 Targeting GROUND-BASED SEEDING Seeding sites are fixed, that is, not mobile. Multiple sites are required. Site selection is based upon prevailing wind direction(s). The altitude required for the sites depends upon the frequency (how often) and strength of inversion layers, and also the prevalent wind speeds. AIRBORNE SEEDING Seeding can be conducted wherever needed, regardless of wind direction. Flight paths and altitudes are based upon current (real-time) wind directions and speeds. Seeding duration can be limited by the ability of the aircraft to fly in icing conditions.

25 Terrestrial Seeding Seeding sites are fixed, that is, not mobile. Sites, once selected, are usable for a fixed range of seeding directions, for example, southwest through northwest. Sites are also usable for only for a fixed range of wind speeds. If the winds are too weak (A), seeding agent will not reach the cloud SLW quickly. If too strong (B), the snow created by seeding won t have time to grow enough to precipitate on the mountain. A Weak winds B Strong winds..

26 Terrestrial Seeding Seeding sites are fixed, perhaps as shown here. Siting considerations include: Distance from the mountain crest (blue line). Spacing between generators. When wind is strong, spacing must be less because seeding plumes have less time to spread. When wind is weak, spacing can be greater because seeding plumes have much time to spread. If generators are too far from the crest, especially if at low elevations, the chance of seeding agents reaching the SLW can be significantly reduced.

27 Airborne Seeding Seeding paths are flexible, limited only by the mountains themselves. When wind is strong, the clouds are generally broader, so seeding legs are flown at greater distance from the crest (blue line), to allow more time for ice crystal growth. When wind is weak, clouds are narrower, so legs are flown closer. There is no concern about seeding agent being trapped in valleys, as the aircraft fly above any inversions.

28 Which Mode is Better? Terrestrial Requires multiple sites, often at higher elevations. Less expensive, per site. Can be operated steadily for long periods of time (even a day or more). Seedable wind directions are limited. Slow seeding rate, typically 25 grams per hour. Solutions are often burned; pyrotechnics can be used if greater rates are needed. Targeting may be less certain. Airborne Typically only one aircraft per mountain range. (twin-engine, known icing) Can be expensive, especially if using cloud physics and/or aerosol instrumentation. Higher seeding rates necessary because of aircraft speed, typically 35 grams per minute or higher. Pyrotechnics are used, seldom solution. Aircraft may periodically have to stop seeding for short periods to deice the airframe.

29 So which to Use? Both systems can be effective. Selection of effective terrestrial sites and aircraft paths are critically important. This can be improved by numerical modeling. Plume transport can be verified by airborne seeding plume measurements. The best terrestrial sites often are well removed from commercial electrical power, and so must be powered by solar and wind energy, with battery backup. Control is possible via satellite.

30 Opportunity Recognition WHEN ARE CONDITIONS RIGHT? Cloud must contain liquid water [microwave radiometer]. Cloud must be cold enough. [sounding (weather balloon) or aircraft, or numerical modeling.] If ground-based seeding is used, the SLW must be within 1 km above the mountain crest. [difficult to know with certainty] Flow must be over the mountain crest, not around it, at least at the seeding elevation or altitude. [numerical modeling]

31 Even the basics get complicated! CONCLUSIONS Opportunity recognition allows us to begin seeding during favorable conditions. Effective targeting is critical. Airborne and terrestrial-based seeding both have their place. (They can even be used at the same time!) An extensive research project examining this kind of seeding is presently concluding in Wyoming, USA. That evaluation is both statistical (precipitation analysis) and physical (in situ and remote cloud measurements), and numerical modeling. (I cannot discuss results because the project does not conclude until January Ask me then.)

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